This invention relates to blind fasteners for securing sheet material together and, in particular to a blind bolt having improved shear, bending and tension values.
Blind fasteners have become particularly useful in securing sheet materials where it is impossible to access the underside of the material. Typically, such fasteners are inserted into an aperture through the material and include a threaded stem for engaging a fastener. Rotation of the stem draws an expansion member into engagement with the underside of the material. Expansion may be aided by a nut body which forces the expansion member radially outward to engage the underside. Upon complete expansion to fully clamp the sheet material, the exterior end of threaded stem may be broken away to eliminate any protrusions from the exterior surface.
Such fasteners have wide application in aircraft and space vehicle assembly. However, the vibrations and sonic fatigue such vehicles are subject to causes loosening of the fasteners.
Previously available blind bolts comprise an externally threaded core bolt which is positioned within a hollow bolt and an internally threaded collar is threaded onto the core bolt. The core bolt also includes a breakneck along its length. The blind bolt is installed by placing the hollow bolt within an aperture in one or more workpieces and rotating the core bolt with respect to the collar. Rotation of the core bolt moves the collar up the hollow bolt causing it to expand radially. Further rotation of the core bolt continues to move the collar towards the head of the hollow bolt until it abuts the underside of the structure. The collar then stops moving axially and begins to expand radially wherein continued rotation causes the collar to reach a maximum diameter and an increasing compressive load on the structure. The resistence increases until it reaches a point when the breakneck on the core bolt shaft fractures in torsional loading.
A problem with previously available blind bolt designs is that they require a slight deformation in the grip area of the hollow bolt in order to captivate or offer rotational resistence to their core bolt design. The purpose of crimping or deformation is to prevent unloosening or separation of the components. This added friction often complicates the range of acceptable breakneck fracture levels. Another problem with currently available blind bolt designs is that if during installation should there be a tensile load sufficient enough to fail the core bolt, the collar would be free to fall from the hollow bolt, thus causing a structural weakness.
Consequently, a need exists for an improved blind bolt design with improved shear bending and tension values and which eliminates the problems associated with previously available blind bolt designs.